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News Analysis After all the time, money, anguish and work to return the space shuttle fleet to flight, the US space agency still has one lingering question to resolve. Can a shuttle mission be sent to save Hubble?

If not for the 2003 Columbia disaster, the fifth servicing call to the Hubble Space Telescope probably would have happened by now.

That mission would have left the 16-year-old observatory with fresh batteries, replacement gyroscopes for steering and two new science instruments.

Such upgrades could keep Hubble on the cutting edge of astronomical research, delving into questions such as the nature of the mysterious dark energy that seems to be driving the universe's expansion and what the universe looked like as its earliest structures emerged after the Big Bang.

But after the Columbia accident shuttle flights that do not go to the space station, where astronauts can seek shelter in case their ship is too damaged to fly home, were considered too risky and the Hubble house call was cancelled.

NASA looked into a robotic servicing mission instead, but found the cost and complexity of such a mission too daunting.

Meanwhile, public outrage over NASA's decision to let the popular space telescope fester, culminating in a pledge by the new head of NASA to re-consider flying one last flight to Hubble.

Now, with three post-Columbia flights complete, the time has come to decide Hubble's fate.

NASA is considering if the safety upgrades implemented after the accident offset the risk of flying with few options available to save a stranded shuttle crew.

"Hubble is definitely sort of the comeback kid, so hopefully it will be able to do it one more time," says Professor Adam Riess, an astronomer with the Space Telescope Science Institute in Baltimore.

"I would expect that this is the final meeting," he adds. "Now it's a matter of coming up a with a decision."

Shuttle program workers have certainly done their homework.

The first order of business was to fix problems with debris falling off the ship's fuel tank during launch.

A chunk of insulating foam broke off Columbia's tank during lift-off and smashed a hole in the wing's heat shield.

As the shuttle attempted to return to Earth 16 days later, superheated atmospheric gases blasted into the breach, destroying the shuttle and killing the seven astronaut crew.

NASA's first tank redesign, tested during a July 2005 flight of shuttle Discovery, failed to prevent dangerously large pieces of foam from falling off the tank. A second redesign was tested a year later, and this time the tank was deemed safe enough to resume flights.

The shuttle program also demonstrated how the ship's robot arm and newly added extension boom could be used during flight to scrutinise the shuttle's heat shield for damages.

Astronauts also tested using the boom as a work platform in case repairs were needed to fix heat shield holes before attempting atmospheric re-entry.

But there is no getting around the issue of flying without an orbital safe. Hubble's orbit is too far from the station for a shuttle to get there.

Theoretically, NASA could severely cut shuttle power and keep a crew in orbit for several weeks until another shuttle could be launched on a rescue mission. But there is no guarantee the flight could be launched on time or that another problem might not also strike the rescue ship.

"You just don't have the orbital lifetime on a Hubble mission to be able to get another vehicle launched," says deputy shuttle program manager John Shannon.

"We're going to have to go into the Hubble decision not counting on the launch-on-need vehicle," he says.

"That's the difficult question the agency's going to have. Do we have enough confidence in the [tank] design? Do we have enough confidence in our inspection and repair to be able to do that? I think that's where the discussion is mostly going to lie."

Another issue is whether to risk a shuttle and crew on a Hubble flight when NASA is under a strict deadline to finish the space station before 2010 when the shuttles are to be retired.

The agency needs at least 14 more construction missions and wants two additional flights beyond that to deliver heavy equipment and spare parts to the outpost.

In a recently released flight schedule, agency managers set aside a Hubble servicing mission for April 2008.

Station assembly, which resumed with last month's flight of shuttle Atlantis, would be complete in July 2010 under the new manifest.

Without a servicing mission, astronomers expect Hubble to last only another two or three years before its batteries give out. Even before then, the finicky gyroscopes could break or the cameras could fail.

If it gets upgrades, Hubble should last until at least 2013. That means it could still be operational when its sharp-eyed infrared replacement, the James Webb Space Telescope, arrives in orbit to begin its five-year mission.

The Webb observatory, which is not designed for servicing by astronauts, is scheduled for launch in June 2013.